1. Field of the Invention
The present invention relates to an image processing device, such as a digital camera or the like, and more particularly, it relates to an image processing technology capable of improving S/N after compressing/expanding conversion and obtaining a favorable gradation characteristic.
2. Description of the Related Art
Recently, with the advance of image input equipment, such as a digital camera or the like, it has become possible to take in image data with high accuracy. Such image data taken in with high accuracy can be stored in a storage device, and a user can easily output the image data to an output device, such as a monitor, a printer or the like, by a personal computer or the like.
When storing image data, the image data is sometimes stored as digital signals of 12-14 bit per pixel. For example, if it is 12 bits, the number of gradation is 4096. In this way, the image data of 12-14 bits taken in with high accuracy is stored in a storage medium after it is not compressed or reversibly compressed.
In this case, if the image data of 12 bits per pixel is stored without being processed, the amount of the data becomes enormous. Therefore, the image data is stored in the storage medium after it is compressed.
As a method for storing images shot by an image input device, such as a digital camera or the like, there is a method for anticipating that the lower-order bits of image data are camera noise or the like and cutting them away. As the method for cutting lower-order bits, a method for cutting away the entire pixel and a method for modifying lower-order bits to be cut away case by case taking into considering the amount of noise are proposed.
Japanese Patent Application No. 2000-244922 discloses a method for cutting away lower-order bits, using the number of bits corresponding to a valid data value and setting the number of bits for each pixel (method for correlating lower-order bits to higher-order bits).
However, if image processing is applied to compressed data obtained by cutting away lower-order bits after it is being expanded, the image processing is applied with the reduced number of gradation due to the cutting of bits. Therefore, when compared with a case with no cutting of bits, an unnatural change of level (tone jump) occurs in the edge and gradation changing part of an image.